I learned one thousand kanji in January.
Which do you think there are more of: Americans who were born in January or Italians who were born in June?
It’s not bad to eat ice cream in January.
Since the kanji 月 mean both moon and month (because a moon cycle is about a month, presumably), what is the number one month? In English we have an actual name for it, and that name is January.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. This is a specific month, so it uses the がつ reading.